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<p>单击按钮将段落中的第一个 a 替换成 b：</p>
<p id="demo">a a!a a!</p>
<button onclick="myFunction()">点我</button>
<script>
function myFunction(){
	var str=document.getElementById("demo").innerHTML; 
	var n=str.replace("a","b");
	document.getElementById("demo").innerHTML=n;
}
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